It is known that certain species of fish can and do migrate back and forth between fresh and saltwater environments as a part of their normal lifecycle. This can happen, for instance, during spawning. Common examples include salmon and eels among others. Other fish simply seasonally migrate from saltwater to brackish (less salty) water environments. Common examples of these fish include shad and rockfish. Still other fish may simply migrate in fresh water in upstream and downstream directions, all in fresh water, depending on their biological lifecycle. These fish include Asian carp.
A widely used method to capture fish includes indiscriminate netting and longline fishing. These fishing methods can result in wasteful bycatch—the catching of fish that are of no commercial importance to that particular fisherman.
Man-made structures such as dams can reduce or prevent the normal migration of fish between various bodies of water including saltwater and freshwater environments. Known solutions to overcoming these physical barriers like dams include fish ramps and efforts that provide a pathway for migrating fish around the man-made structure.
Migrating fish such as eels, and other fish generally, may have high sensitivity to water conditions. These fish can apparently detect subtle changes in, for instance, water salinity, oxygen, temperature, nutrition and rate of current flow. Depending on the age of the eel and the season, eels detect changes in water to move up into freshwater environments, or alternatively, downstream to brackish and ocean water environments. Eels innately swim in appropriate directions upstream and downstream according to their age and the season.